Asked by: Richard H Gray, Northern Ireland
Seismometers vary in design, but most modern ones use a heavy mass connected by springs to a frame. Often a magnetic field is applied, creating a force that maintains the position of the weight. The frame is firmly attached to a geologically solid footing such as bedrock. Then, in a quake, the device vibrates with it. Through a negative feedback system, the magnetic field regulates itself to keep the mass still as the frame vibrates around it. The shifting magnetic field draws an electrical current that varies accordingly. It’s this signal that the seismometer measures. The components are chosen to give a linear relationship between ground movements and seismographic measurements. To calibrate the device, outputs are measured for a known range of displacements and frequencies.